Leukocyte recruitment is a crucial part of the immune response to infectious and harmful agents. Selectins are adhesion molecules expressed on leukocytes, endothelium, and platelets, which play a major role during the recruitment cascade, specifically during rolling. The selectins, L-, E-, and P-selectin, are proteolytically cleaved from the cell membrane and are detectable as soluble forms in the plasma under physiologic conditions. The concentrations of soluble selectins can vary considerably under different pathological conditions, making soluble selectin measurements valuable diagnostic markers for clinical scenarios. However, except for involvement of sP-selectin in hemostasis, little is known about the function of the soluble selectins. Similarly, except for the importance of Lselectin shedding in leukocyte rolling, the physiologic function of selectin shedding remains enigmatic. Furthermore, the proteases responsible for physiologic selectin shedding are unidentified, save for TNF-a. Cleaving Enzyme, TACE, which has been shown to cleave L-selectin. This research will make combinatory use of the in vivo assay of intravital microscopy with in vitro assays of flow cytometry , histology, protein chemistry, and genetic manipulations of cells and mice to study the physiology and mechanisms of selectin shedding and the function of the resulting soluble selectin fragments. Gene targeted mice deficient for L-, E-, and P-selectins, the ACT version of P-selectin, combinations thereof, and TACE are available for this project. Further, chimeric mice generated by transplantation of embryonic progenitor cells from the TACE-/- into WT or selectin deficient mice will be used for this project. Our overall hypothesis is that shedding of selectins significantly impacts inflammatory and hemostatic outcome. Therefore understanding the mechanisms and the role of selectin shedding in inflammatory leukocyte recruitment would make them attractive targets for future clinical interventions.